This article is part of Yahoo's 'On This Day' series
The one thing that no worker in a nuclear plant wants to see is a flash of blue light.
The blue light indicates a nuclear chain reaction, similar to what happens inside a nuclear weapon.
For nuclear plant worker Hisashi Ouchi, a blue flash above a vat of uranium spelt a death sentence in 1999, when he and his colleagues triggered what was then the worst nuclear accident in Japanese history.
He also gained the unenviable record of being labelled the ‘most radioactive man in history’ in the accident on September 30, 1999 (although, technically, he’s the most-irradiated).
Ouchi and his colleagues worked in a small private-run plant in Tokaimura, and were preparing uranium for use as reactor fuel.
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They made several fatal mistakes, including not wearing the correct protective gear, and transporting chemicals containing uranium in buckets.
In the vat was 16kg of uranium: the maximum that should have been together was 2.4kg.
Combining that much of the highly radioactive substance caused a chain reaction which led to criticality, and a release of deadly neutrons.
The workers rapidly lost consciousness and alarms wailed inside the plant as radiation levels rocketed to 4,000 times the normal level.
The local area was evacuated, with many not having even been aware that the small unassuming building was a nuclear facility.
Peter Glaskin, the headmaster of an international school just a mile from the blast said, ‘'I thought something was up when I saw lots of helicopters buzzing overhead, but I had no idea that it was so close.”'
A third of a million people were told to stay indoors, as Ouchi and his colleagues were transported to hospital. His colleague Masato Shinohara also died months later from radiation exposure.
But Ouchi, who had been ‘draped over’ the tank when the incident occurred, absorbed far more radiation, a record 17 Sieverts.
Ouchi had severe radiation burns over his entire body, and was treated in intensive care, using skin grafts, blood transfusions and stem-cell treatments.
After a week of treatment, he said to doctors, “I can’t take it any more. I am not a guinea pig.”
His torment continued for months, with local news reports claiming he ‘leaked’ 20 litres of fluid from his body every day, and was ‘crying blood’.
Watch: The never-ending challenge of radioactive water at Fukushima
Despite begging doctors, “Stop it,” he was resuscitated after suffering heart attacks, at the request of his family.
Eighty-three days later, he died.
More than 600 plant workers and emergency personnel were exposed to radiation in the incident.
The plant operator, JCO, had its credentials for running a nuclear plant cancelled for mishandling nuclear materials.
In 2011, another nuclear disaster struck Japan.
The Fukushima Daiichi nuclear disaster was triggered by an earthquake, and is considered the second-most severe nuclear accident in human history, after the 1986 Chernobyl disaster.
As of 2021, there are nine nuclear reactors still operating in Japan.